Two problems that occur in communication systems are referred to as dropout and fade. These problems arise in multi-path systems, wherein a signal proceeds to a receiver along a plurality of communication paths of different transmission lengths, with the received signal being a composite of different signals having randomly varying delays and amplitudes.
One approach to dealing with these multi-path problems is described in the article "A Communication Technique For Multi-Path Channels" by R. Price and P. E. Green, Jr. (Proceedings of the IRE, March 1958, pages 555-570). These authors describe a system that performs a continuous, detailed measurement of a single multi-path characteristic. This knowledge is then exploited to combat the effects of multi-path fading. Specifically, selective fading is treated by first detecting the individual echo signal, and then adding the signals using an appropriate algebraic correlation technique. Inter-symbol interference is dealt with by reinserting different delays into the various detected echoes. This system is referred to as a "Rake" system.
In general, the purpose of the Rake system is to equalize the signal dispersion from a single transmitter through an ionospheric multi-path medium, thereby reducing the inter-symbol interference and improving the signal-to-noise ratio (SNR). The Rake system is applicable to a multipath system which has a large number of incremental delays resulting from the different conditions in each of the paths. It necessitates lengthy tapped delay lines with a large number of taps, to compensate for the equally large number of multiple paths. Furthermore, the Rake signals have no significant Doppler spread, and the length of the tapped delay line is significantly shorter than the length of an information bit. As a result, the Rake system is not suitable for use in communication systems where the time and the frequency of the received signal varies greatly.
The journal article "Increased Capacity Using CDMA For Mobile Satellite Communication", by K. S. Gilhousen et al. (IEEE Journal on Selected Areas In Communications, vol. 8, No. 4, May 1990, pages 503-514) describes the performance of a spread-spectrum (SS) Code Division Multiple Access (CDMA) communication system in a mobile satellite environment. These authors mention (page 506) that multiple satellites provide a further method for improving CDMA capacity by combining the coherent signals that are transmitted between a terminal and all satellites in view.
In U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters", K. S. Gilhousen et al. describe a communication system that provides marginal isolation between different user communication signals. The marginal isolation is provided by simultaneously generating multiple steerable beams using a single omni-directional antenna with polarization enhancement. Power control devices are used to adjust the output power, either in accordance with their input activity level, or with a minimum allowable power level for maintaining a communication path. In col. 32 it is stated that satellite or terrestrial based repeaters provide a variety of communication paths which compensate for Doppler Shifts and multi-path problems, to provide an increased reliability of the communication path.
In U.S. Pat. No. 5,233,626, issued Aug. 3, 1993, S. A. Ames discloses a "Repeater Diversity Spread Spectrum Communication System" (incorporated herein by reference in its entirety). This communication system utilizes multi-path spread spectrum radio techniques, and which utilizes at least one linear communications repeater for each of those paths. In this disclosure, the term "linear" as relating to repeaters is defined as a repeater which is powered in such a manner as to provide an output which is within the linear range of all of its components, and will not experience excessive signal distortions within the power range applied to the repeater. In the Ames system, all of the repeaters are normally configured to transmit at their full power even when one of the repeaters is blocked or otherwise unusable. This continuous, full power transmission expends considerable repeater energy, which is of particular concern in satellite based repeaters.
In U.S. Pat. No. 5,093,840, issued Mar. 3, 1992, D. L. Schilling discloses an apparatus for adaptive-power control of a spread-spectrum transmitter of a mobile station operating in a cellular communications network. This adaptive power control only regulates a signal over a single path, and is not applied to more than one path.
In U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, Gilhousen illustrates a multi-path CDMA transmission system, configured for cellular telephones, incorporating a power control system which adjusts the strength of a transmission signal (in cell-site 12), depending upon variation in the signal power. There is only one transmitting antenna which transmits the signal to each cell-site 12, or repeater.
The prior art illustrates a progression of multi-path spread spectrum systems capable of providing increasing levels of signal transmission quality. However, none of the above systems utilizes a multi-path spread spectrum system which also provides a mechanism to limit future transmissions of signal copies over a particular communication path based on signal quality.
One of the prime considerations in satellite applications is energy expenditure and conservation. If a multiple communication path system (the term "communication path" is defined in this disclosure as a combination of a first transceiver, a second transceiver, and at least one repeater) could be constructed that consumes less average overall energy while transmitted data with equivalent reliability, then smaller and/or lighter repeaters could be utilized, which would result in smaller and/or lighter satellites.